In the prior art, research has been carried out into converting light (a fundamental light wave) emitted from a laser medium, such as an Nd:YAG laser or an Nd:YVO4 laser, into visible green light (a harmonic wave) and further converting the green light into ultraviolet light, by wavelength conversion using a non-linear optical effect. Consequently, a large number of wavelength conversion laser light sources which yield visible laser light or ultraviolet laser light have been developed and put into practical use. This visible laser light and ultraviolet laser light is used in applications such as light sources for material laser processing, laser displays, and the like.
FIG. 13 is a diagram showing a compositional example of a conventional wavelength conversion laser light source that employs Nd:YVO4, which is a monocrystalline material.
The wavelength conversion laser light source 100 shown in FIG. 13 is an end-pumped type of laser light source in which pump light is input from an end face of the laser medium. A YVO4 crystal, which is a monocrystalline material, is used for the solid laser medium 105. The pump light PL is generated by a pump light source 101, converted into parallel light by a collimating lens 103, and is then condensed into a solid laser medium 105 in a resonator 109 by a condensing lens 104.
A high-reflection optical film 105a which reflects 1060 nm-band light is formed on an end face of the solid laser medium 105, on the side where the pump light PL is incident, and a high-reflection optical film 106a which reflects 1060 nm-band light is formed on an end face of a concave mirror 106. The resonator 109 is composed by the high-reflection optical film 105a and the high-reflection optical film 106a. 
Here, an anti-reflection optical film (not illustrated) is formed on the end face 105b which is the surface of the solid laser medium 105 on the surface opposite to the wavelength conversion element 107, and on both end faces 111 of the wavelength conversion element 107. In other words, an anti-reflection optical film is formed on the surface of the solid laser medium 105 opposing the wavelength conversion element 107 and the surface of the wavelength conversion element 107 opposing the solid laser medium 105. Since the light resonates between the high-reflection optical film 105a formed on the solid laser medium 105 and the high-reflection optical film 106a formed on the end face of the concave mirror 106, then the resonator 109 operates as an optical resonator and laser light in the 1060 nm waveband is generated.
In this case, when the 1060 nm-band light thus generated passes through the wavelength conversion element 107, the wavelength of the 1060 nm-band light is converted to obtain output light OL at 530 nm, which is half the wavelength. The 530 nm output light OL thus converted is output externally from the end face of the wavelength conversion element 107 and via the concave mirror 106. The solid laser medium 105 is held by a laser medium holding tool (not illustrated).
Normally, in order to convert the wavelength of light using a wavelength conversion element, it is necessary for the 1060 nm-band light, which is the fundamental light wave, to be linearly polarized light. Since the YVO4 crystals which are used as the solid laser medium 150 shown in FIG. 13 are a material having optical anisotropy, then by aligning the input and emission faces of the solid laser medium 105 in a plane which contains both the a axis and the c axis of the crystal axes, it is possible to obtain the generated 1060 nm-band light as linearly polarized light.
As described above, in a wavelength conversion laser light source, it is possible to carry out wavelength conversion based on a non-linear optical effect only in respect of light having a certain particular direction of polarization, and therefore in order to improve the output of the laser light source, it is important that the light emitted from the solid laser medium is linearly polarized light. With a material having anisotropy in the crystal structure, such as the Nd:YVO4 described above, it is possible to directly obtain polarized light simply by selecting the axial orientation of the monocrystalline structure.
However, with an isotropic monocrystalline material, such as Nd:YAG, or a ceramic laser medium, even if the axial orientation of the solid laser medium is selected, it is not possible to ensure that the light emitted from the solid laser medium is directly polarized light.
In this way, in a YAG crystal or a solid laser medium using ceramic material, since there is no optical anisotropy, it is not possible to obtain directly polarized light if the composition shown in FIG. 13 is used directly without modification. Therefore, various compositions have been proposed in which a wavelength plate is inserted into a resonator in order to enable wavelength conversion, even when using a solid laser medium which does not have anisotropy.
For example, in a composition of a wavelength conversion laser light source such as an internal resonator type of wavelength conversion laser light source, there are examples where it has been proposed to employ an optical component such as a ¼ wavelength plate and to use all of the polarization directions emitted from the solid laser medium; for instance, there are the wavelength conversion laser light sources disclosed in Patent Documents 1 to 3 described below.
FIG. 14 is a schematic drawing showing a composition of a conventional first wavelength conversion laser light source which uses a wavelength plate, FIG. 15 is a schematic drawing showing a composition of a conventional second wavelength conversion laser light source and FIG. 16 is a schematic drawing showing a composition of a conventional third wavelength conversion laser light source using a wavelength plate.
Firstly, in Patent Document 1, as shown in FIG. 14, pump light generated from a pump light source 101 is converted into parallel light by an object lens 102, and is then input to a solid laser medium 105 via a reflecting mirror 110, and a resonator 204 is constituted by the reflecting mirrors 110 and 111. Furthermore, two wavelength conversion elements 107 and 108 and a wavelength plate 203 are provided inside the wavelength conversion laser light source and the direction of polarization of the harmonic laser beam generated by converting the light wavelength by one wavelength conversion element 107 is rotated by the wavelength plate 203 so as to coincide with the direction of polarization of the harmonic laser beam generated by converting the light wavelength by the other wavelength conversion element 108. Consequently, there is little variation in the direction of polarization with respect to temperature change in the device, and it is possible to obtain a laser beam having a stable output.
Thereupon, in Patent Document 2, as shown in FIG. 15, the pump light generated by the pump light source 101 is converted into parallel light by a collimating lens 103 and is then condensed in the solid laser medium 105 by the condensing lens 104, and a resonator 206 is constituted by the high-reflection optical film 105a and the reflecting mirror 111; output light OL which is a second harmonic wave is output externally via the reflecting mirror 111 and the wavelength filter 210.
Here, the resonance mode in the resonator 206 of the wavelength conversion laser light source is constituted by two intrinsic polarization modes in which the laser light of the fundamental wave is mutually orthogonal. By resonating operation in a randomly polarized state where there is no correlation in the phase relationship between these two intrinsic polarization modes, and also inserting a λ/4 wavelength plate 203 in the resonator 206, exchange of energy via the generation of the second harmonic wave does not occur between the two intrinsically polarized light beams which are mutually orthogonal. Thereby, it is possible to generate second harmonic laser light which is sufficiently stable for practical application, readily by a simple composition.
Next, Patent Document 3 proposes preventing positional displacement due to the heat of respective optical elements constituting a resonator by arranging the optical elements independently on a prescribed substrate, and discloses that, of these optical components, a resonator 207 is constituted by a high-reflection film 205a and a high-reflection optical film 107a, as shown in FIG. 16, and the resonator 207 uses an optical component 205 which combines a wavelength plate and a lens.
A method such as that described above has been proposed already and with the conventional wavelength conversion laser light source described above, it is possible to raise the conversion efficiency from the fundamental light wave to the harmonic wave, and it is possible to stabilize the output.
However, it is known that, if a wavelength plate is inserted into the resonator in order to stabilize the output from the wavelength conversion laser light source, and if the input pump light is made large in order to raise the light output from the wavelength conversion laser light source, as in the prior art example described above, then problems occur in that the light output that ought to be achieved according to the design is not obtained, or the light output becomes instable. More specifically, it is clear that the effect of stabilizing output is not sufficient if the contents of the prior art example described above are simply implemented directly.
FIG. 17 is a plot diagram of the output characteristics of a conventional wavelength conversion laser light source, in which the horizontal axis represents the input intensity of the pump light and the vertical axis represents the output intensity of green light, which is the output light of the wavelength conversion laser light source. FIG. 18 is a schematic drawing showing a composition of a conventional wavelength conversion laser light source from which the output characteristics shown in FIG. 17 were measured.
As shown in FIG. 18, the pump light PL output from the pump light source 101 passes through the collimating lens 103 and the condensing lens 104 and is input into the solid laser medium 105. The high-reflection optical film 105a which reflects the 1064 nm waveband for constituting a laser resonator is formed on one end face of the solid laser medium 105, and a laser resonator is composed by the high-reflection optical film 105a and the high-reflection optical film 106a of the concave mirror 106. Upon passing through the wavelength conversion element 107, the 1064 nm light (fundamental light wave) which is generated inside the laser resonator is converted to 532 nm, which is half the wavelength, and is radiated externally as output light OL from the concave mirror 106. The wavelength plate 203 is inserted between the solid laser medium 105 and the wavelength conversion element 107.
The plot diagram shown in FIG. 17 was obtained as a result of measuring the output characteristics of a conventional wavelength conversion laser light source composed as described above. As shown in FIG. 17, when the pump light input is 1.5 W or greater (when the green light output is 200 mW), the output starts to become instable and if the pump light input becomes 3 W or greater (when the green light output is 500 mW or greater), then it is evident that, in addition to instability of the output, the green light output diverges by approximately 15% from the predicted output value based on calculation (calculation value).
Patent Document 1: Japanese Patent Application Publication No. H5-167166
Patent Document 2: Japanese Patent Application Publication No. H1-220879
Patent Document 3: Japanese Patent Application Publication No. H8-56042